SYNTHESIS OF COUMARINS AND DERIVATIVES .1. BIOMIMETIC SYNTHESIS OF ESCULETIN AND HALOGENATED DERIVATIVES

Esculetin (1) and the novel compounds 5-chloroesculetin (5) and 5-bromoesculetin (6) were obtained from a light-induced cyclization of trans-caffeic acid (3) catalyzed by [FeNa(edta)] and/or H2SO4, HCl, or HBr (Scheme 1). The experimental conditions for trans-cis-isomerization of the cinnamic-acid derivative 3 and subsequent nonenzymatic cyclization were described. The photoperiod and the presence of air and iron-chelate catalyst are shown to be important parameters that markedly affect yields. The reactions probably occur by a free-radical mechanism involving a photo-initiated one-electron redox process (Scheme 2).